Systems for carrying out the combustion of material are employed by numerous industrial entities, ranging from those engaged in chemical production to power generating plants. As concern has intensified over the discharge of combustion products into the atmosphere, techniques have been sought for effectively monitoring combustion processes. By monitoring process parameters such as combustion air, fuel, and exhaust gases, higher combustion efficiencies can be developed, for example by optimizing fuel-to-air ratios.
An increasingly important aspect for combustion monitoring concerns mass balance analysis, where the mass transport of gas, fuel and other materials entering combustion and exiting therefrom as stack gas is evaluated. Such evaluations are required, for example, in the burning off or combustion of dangerous chemicals. For many applications, the gases being monitored are at quite high temperatures at each side of the combustion region. Combustion input air may be pre-heated to reach temperatures up to about 800.degree. F., while exhausting stack gas may reach temperatures extending above 1500.degree. F.
Monitoring procedures typically rely upon the positioning of relatively small flow meter probes within the gas stream. These instruments, while measuring a local velocity vector of the moving gas, can be calibrated to measure mass transport. One particularly accurate metering technique employs an arrangement wherein a reference temperature measuring probe is associated with a locally heated temperature measuring probe and the conductance of heat from the latter due to gas movement is converted to a velocity readout. Temperature sensing probes used for such purposes may, for example, be resistant temperature devices (RTD) formed of platinum materials or the like. Generally, such RTD devices, when tested under static conditions, will provide accurate temperature evaluations up to about 1600.degree. F. It has been found, however, that such conventional flow meter probes exhibit signal loss when called upon to monitor velocity components of gases at temperatures above about 800.degree. F. Thus, a need has arisen for a flow meter device capable of measuring gas flow at elevated temperatures.